| Day | Date | High/Low | Forecast |
| Mon | 26th | 80/63 | Sunny day. Clear evening. |
| Tue | 27th | 79/62 | Sunny day. Clear evening. |
| Wed | 28th | 77/62 | Chance of some early morning low clouds; Otherwise, mostly sunny day. Clear evening. |
| Thu | 29th | 76/62 | Chance of some early morning low clouds; Otherwise, mostly sunny day. Clear evening. |
| Fri | 30th | 77/63 | Chance of some early morning low clouds; Otherwise, mostly sunny day. Clear evening. |
Synopsis
27 days until the start of autumn (astronomically speaking)…Like the first half of last week, high pressure is the dominant, weather force over most of the West. Last week, it wound up warmer than I expected. A nearly, surface, off-shore gradient and decent subsidence (sinking air motion) warmed the air even at most beaches (Santa Monica pier reached 79 degrees at height of warm spell; UCLA peaked at 87 degrees). There are some similarities in the weather pattern this week, but high pressure aloft is weaker this week than last week. So, most locales haven’t warmed as much as last week (so far). Even at its peak (tomorrow for well inland areas), no triple digit readings west of the mountains are anticipated. Close to the coast, it may cool a tad tomorrow, if surface on-shore flow picks up as the numerical models predict.
By midweek, a very weak area of low pressure aloft should form just west of the California. In addition, a trough should be moving into the northern Rocky mountain states from the west. This should weaken and displace the current high pressure to the south of our region. A general though mostly, minor cooling trend should ensue. Thursday should be the coolest day this week for most areas (slightly cooler than normal). A little rebound in temperatures should follow by the beginning of the Labor Day weekend (high pressure makes another move back into the Southwest).
Last week, a different low pressure trough didn’t promote as much cooling as I expected. This week’s, predicted low pressure will be much weaker (no threat of wet weather in northern California, let alone any snow showers). However, all the models predict the maintenance of a shallow marine layer this week (likely continuing over the holiday weekend). Satellite imagery does show a swath of low clouds west of the coastal islands. So, it’s possible that widespread low clouds could return to the coastal plain by midweek. Whether low cloud coverage ends up haphazard like late last week, I’m unsure. The current, warm ocean temperatures in the southern, inner, coastal waters may be playing some role in disrupting the low cloud field (some spots west of San Diego County reaching the low/mid-70s), not to mention keeping air temperatures from falling significantly. However, a conclusive explanation eludes me.
A separate, new trough is forecast to move inland over the Pacific Northwest and northern California later in the holiday weekend (stronger than midweek trough but still much weaker than last weekend’s trough). It may help increase the surface on-shore flow in southern California (tipping daytime temperatures slightly lower). However, some model forecasts keep the effects of the trough too far to the north to influence Southland weather. In that case, slightly warmer than normal may continue. There are even some scenarios that include better warming than what’s expected today/tomorrow.
Looking farther out in time next week, most model solutions favor some form of weak troughing along or just west of the state. If that scenario verifies, a minor cooling cycle should develop later next week. There are currently no scenarios favoring a moderate cooling for southern California.
Some model solutions show a minor, monsoon “moisture” incursion aloft late this week. It’s very marginal, but a slight risk of a brief, afternoon thunderstorm over the southern mountains or eastern desert is possible. Next week, some model solutions show a somewhat greater, moisture incursion by midweek. Isolated, brief-lived thunderstorms could occur over the southern mountains/eastern deserts. Still, a slight error in predicted wind flow could easily result in no “monsoon” incursion into the state.
Next issued forecast/synopsis should be on Tuesday, 3 September.